EP0233147B1 - Method and apparatus for continuous twin-roll casting - Google Patents

Abstract

A flow of coolant is injected or blown into spaces bounded by a cast strip the point of minimal separation between the rolls, and the rolls themselves. The coolant is injected or blown by means of nozzles disposed on both sides of the strip. In the case of an asymmetric emergence of the strip due to increased adhesion to one of the rolls, the resulting asymmetric conditions cause the strip to continually be redirected into a symmetric position. This stabilizing effect renders it possible to achieve a much greater length of contact between the cast metal and the rolls and consequently to realize the essential increase of the production rate of the casting line.

Description

The present invention relates to a roll casting method according to the preamble of claim 1 and a roll casting system for performing the method.

Since the 1930s, this process has been used industrially with so-called roll casting machines (ROLL CASTER) and has become increasingly important since 1955 (E. Herrmann. «Handbook on Continuous Casting •, 1980 edition, pp. 63-65 and 166-167) .

The strip thickness produced in systems previously carried out is between 3 and 15 mm, mostly 6 to 8 mm. The bandwidth in newer production plants measures between 0.25 and 2 m. With the appropriate dimensioning of the casting rolls, their storage and the drive, however, the bandwidth to be produced has no limits in terms of casting technology, so that bandwidths of 3 to 4 or more meters can also be realized.

By referring to the casting of aluminum as an example. is, with appropriately adapted data, the analogous application of the roll casting method according to the invention also to other materials. particularly applicable to steel.

The casting process was previously mainly used for the production of aluminum strips. whereby an hourly production of 900 to 1200 kg / m strip width is achieved, depending on the strip thickness produced and the alloy processed. The strip produced thereby leaves the casting nip formed by the two rollers at a speed, generally referred to as the casting speed, of 0.75 to 1.4 m / min. After leaving the rollers, the temperature of the cast strip is usually between 350 ° and 400 ° C.

The casting direction can be arbitrary. Plants are known with which casting is carried out vertically upwards, horizontally or obliquely upwards or downwards.

The rollers are equipped with a cooling system. so that the absorbed heat is dissipated by a coolant. So far, the internal roller cooling has prevailed in that the rollers have channels through which the coolant flows within an attached roller shell. However, external systems can also be used by directly contacting the roller surface with the coolant (Sir Henry Bessemer, 1846).

It is in the interest of every user of the casting process to achieve the highest possible production per unit of time, i. H. operate the plants at the highest possible casting speeds. The condition is that the cast material solidifies over the entire strip cross-section when leaving the rolls, otherwise the liquid metal flows through the rolls, interrupting the casting process or at least severely disrupting it until the melt breakthrough by changing the casting parameters accordingly (reducing the casting speed and / or lowering the metal temperature in the melt feed, cleaning the roller surfaces, etc.) is eliminated.

Since the required contact time between the rolls and the cast material is determined for a given strip thickness and alloy and the same thermal conditions (heat flow), it would be obvious to increase the contact length of the cast material with the rolls by resetting the nozzle (increasing the dimension h, Fig. 1) and increase the casting speed so that the required contact time is still guaranteed. In practice, it has been shown that the solidification of the melt, viewed across the width of the strip, can take place at somewhat different distances after the nozzle emerges. This is due to small variations in the heat transfer as a result of changes in the roll surface occurring over time and / or location, for example due to friction points of the nozzle and / or temperature fluctuations in the cooling water or in the melt or by other circumstances. Therefore, to avoid melt breakthroughs with certainty, there must be a certain distance (distance a. Fig. 3) between the complete solidification of the cast material and the roll outlet.

At today's usual casting speeds mentioned above with a strip thickness of approximately 6 mm (based on aluminum), a distance h of the order of magnitude of approximately 30 mm between the nozzle and roller outlet has proven itself in practice, the middle one Distance a is approximately 12 mm. As a result of the aforementioned reasons, this distance, viewed over the width of the band and over a period of time, can vary between approximately 8 and 16 mm.

This means that a rolling effect takes place after the metal has fully solidified. If the roll diameter is 600 mm, for example, then a = 12 mm results in an average rolling degree of 7.4%, with a local minimum of a = 8 mm, a rolling degree of 3.4% and 12.4% in the maximum at a = 16 mm. With this rolling effect on dry, unlubricated rolls, with a high temperature of the roll surface. experience shows that there is a tendency for the solid strip to stick to the two rollers. The casting tape emerging from these and already having a certain strength or rigidity. hereinafter referred to as the belt, generally has a tendency to leave the rollers in the plane of symmetry. If the adhesive force on one roller is greater than on the other and the difference exceeds a permissible limit, which is mainly due to the flexural strength of the strip at the exit from the casting gap, the strip remains at the higher adhesive force having roller stick and must be released from this with force, which is usually done by provided belt wipers or correspondingly high belt tension. This severely impairs the quality of the tape, making it unusable for most uses in today's quality requirements. By spraying the rollers with a volatile liquid such as suspended graphite. Molybdenum disulfide, boron nitride, magnesium oxide etc., which act as a release agent, can to a certain extent counteract the risk of sticking.

If the casting speed is, for example, 1.2 m / min., There is an average contact time between the casting material and the rolls of 1.5 s for a distance between the nozzle and roll outlet of h = 30 mm (FIG. 3). This is divided into the average solidification time of 0.9 s (solidification distance b = 18 mm, Fig. 3) and 0.6 s average rolling time (rolling distance a = 12 mm, Fig. 3).

If these times are used as the basis for a casting process, it becomes evident that as the casting speed increases, with the time periods remaining the same for the individual phases (solidification, rolling), the relevant distances a. b, and h (Fig. 3) are always correspondingly larger. With the same roll diameter, the degree of rolling and therefore the deformation of the strip inevitably increases with increasing casting speed. Under the increased rolling pressure associated therewith, even with the use of the release agents mentioned above, the adhesion of the strip to the rollers becomes firmer, the permissible difference in adhesive forces on the two rollers also being at least temporarily exceeded considerably. so that the tape sticks to one or the other roller and, as previously described, has to be released with an external force.

The object of the invention is to provide a roll casting process and a roll casting installation for carrying out the process which produces a high stability of the strip at the outlet of the rolls, so that the strip can be released from the rolls even with large different adhesive forces and can be removed freely thus a much greater contact length between the cast material and the rolls, thus enabling a significant increase in the performance of the casting system. At the same time, as intensive secondary cooling of the strip as possible should be achieved at the outlet from the rolls in order to make it more difficult for the melt to break through. The achievement of the object according to the invention is described in the characterizing part of claim 1. Another advantageous procedure is characterized by claim 2. The cooling medium is expediently fed through nozzles arranged on both sides of the belt, the one nozzle wall in each case advantageously being formed directly by the roller surface.

The application of the method according to the invention can be combined with an additional external roller cooling. The cooling zone consists in that the roller surface is wetted, sprayed or blown over a part of the circumference with a cooling medium. In order that the roll surface is cooled directly at the exit from the casting zone by means of the used cooling medium e ^g.

A drying zone adjoining the cooling zone ensures that the roller surface runs dry into the casting zone.

It is possible to add the above-mentioned or other release agents to the cooling medium, which will dry on the roller surfaces and reduce the tape's adhesion to the same.

Drying can be carried out in a known manner by wiping and / or brushing, optionally supported by blowing in cold or warm air, in order to accelerate the residual evaporation of a liquid cooling medium on the roller surface heated by the casting process.

• Figur 1 zeigt einen Querschnitt durch den massgebenden Teil der Anlage,
• Figur 2 zeigt eine teilweise Seitenansicht einer Kühlmitteldüse bei weggenommener Walze, und
• Figur 3 ist ein Teilschnitt zur Erläuterung des Stabilisierungsvorgangs mittels der Kühlmittelzufuhr.

The invention will now be explained in more detail with reference to an exemplary embodiment of the roll casting installation according to the invention shown in the drawing.

• FIG. 1 shows a cross section through the decisive part of the system,
• Figure 2 shows a partial side view of a coolant nozzle with the roller removed, and
• FIG. 3 is a partial section to explain the stabilization process by means of the coolant supply.

The plant shown in FIGS. 1 and 2 has casting rolls 1 and 2 which can be driven in opposite directions in the direction of the arrows in FIGS. 1 and 3. In front of the narrowest point 3 between the two rolls 1 and 2, which point is referred to below as the roll gap or simply gap, there is a pouring nozzle, of which lateral boundary walls 4 are designated in FIGS. Through this nozzle, liquid metal 5 is supplied, which is laterally distributed below the nozzle 4 and cooled on the roller surfaces. The metal then solidifies on the solidification path b and is then rolled on the rolling path a, as described above. The rolled strip 6 exits down through the roll gap 3 and is discharged in a manner which is not shown and is known per se. So far, the system corresponds to the known systems described above.

According to the invention, a coolant nozzle 7a or 7b is now arranged below the gap 3 on both sides of the belt 6. Each of these nozzles has a nozzle body, formed by an inner side wall 8 and an outer side wall 9, two opposite end walls 10, which close off the nozzle body at the ends, and a rear wall 11. Inlet nozzles 12 are provided on the rear wall. through which coolant, preferably water, can be supplied through supply lines, not shown, in a certain amount and at a certain pressure. The two nozzles Bodies are covered at the top by rollers 1 and 2 each forming a boundary wall of the nozzle. For sealing between the nozzle bodies 7 and the roller surfaces can be provided on the edges of the outer side walls 9 and the front, in which sealing rods 14 and 15 e _. are laid. As FIG. 1 shows, these sealing rods lie in grooves 13 with play on all sides, so that the rods are pressed into operation in the sealing position shown in FIG. 1 by the pressure of the cooling water. The sealing bars 14 are straight, and since the friction of the rougher roll surface on these bars is normally greater than the friction of the bars on a machined groove wall, the sealing bars 14 are rotated during operation, resulting in less wear than if they would slide on the roller surface. The sealing rods 15, of course, must of course slide on the roller surfaces. The sealing rods 14 and 15 are made of metal or plastic. The axially extending grooves 13 in the outer side walls 9 open into the circumferentially extending grooves 13 in the end walls 10. The grooves 13 provided in the end walls are closed on both sides by a cover 16 each.

A roller surface and a beveled top section 17 of each inner side wall 8 delimit a nozzle with a slot-shaped nozzle opening 18 that extends axially along a surface line of each roller. Through these openings, a coolant flow can be either tangential or circumferential along the roller surfaces upwards into the between the spaces 19a and 19b formed in the nozzles, the rollers, the gap 3 and the cast strip 6 are injected or blown. The coolant flows out of these spaces downwards through the slot-shaped outlet openings 20a and 20b between an inner side wall 8 and the cast strip 6. These outlet openings 20a and 20b are dimensioned relatively narrow, so that the coolant is stowed in the spaces 19a and 19b and consequently a certain pressure builds up.

In FIG. 1 it is assumed that the cast strip 6 leaves the two rolls 1 and 2 or the gap 3 symmetrically and moves downwards symmetrically between the two nozzles 7a and 7b. There are therefore symmetrical relationships with regard to the coolant flows and the cooling effect, that is to say in particular. that both sides of the belt are cooled to the same extent. The same coolant pressure also prevails in both spaces 19a and 19b, so that the pressure acting on the two sides of the strip is the same. FIG. 3, in which only the uppermost parts of the side walls 8 which form the actual nozzle limitation are shown in a simplified representation, shows the case where the cast strip 6 remains more firmly adhering to the roller 1 than to the roller 2 and thus the rollers or leaves the gap 3 asymmetrically. This naturally also creates an asymmetry of rooms 19a and 19b and the flow and cooling conditions in these rooms. In Figure 3, the coolant flows are indicated by lines 21a and 21b. E t - here it is clear that in the narrower space 19a the adjacent side of the cast strip 6 is cooled over a considerably shorter distance than in the opposite space 19b. This cooling, which is much more intensive on one side, is associated with a considerably stronger contraction on the right side of the cast strip (FIG. 3), a bending moment or a deflection in the direction of the colder strip side being generated by the thermal stresses occurring asymmetrically with respect to the strip center line , whereby the belt continuously separates from an adhesive roller and is controlled against symmetry and thus stabilized.

An additional stabilizing influence is achieved in that the pressure of the coolant in the space 19a rises higher than in the opposite space 19b. It can be clearly seen from FIG. 3 that the outlet opening between the cast strip 6 and the nozzle wall 8 is much narrower on the left side than on the right side. A higher coolant pressure will therefore build up on the left than on the right, and although this higher pressure acts on a somewhat smaller band area than the low pressure 6, a resulting compressive force remains on the band 6 to the right in FIG. 3. The narrowing of the outlet opening 20a also causes a decrease in the flow rate on the left-hand side, which again results in a reduction in the cooling effect on this side. There are thus several influences which cause the band 6 to emerge immediately after its exit from the gap 3 into the plane of symmetry S-S. In addition, there is the increased cooling effect on the rolls and on the belt relatively close to the solidification zone, which further contributes to the fact that the casting speed can be increased.

Corresponding effects can also be achieved in a somewhat different way or intensified through additional measures. Under certain circumstances, nozzles could be provided which also have a nozzle wall extending to the nozzle opening on the side of the rollers. Such an embodiment would have the advantage that no sealant is required between the nozzle and the rollers. However, the execution of such nozzles may present certain difficulties for reasons of space. It is also possible to regulate the coolant flows by suitable means. For example, the position of the strip, the pressure in rooms 19a and 19b or the temperature in these rooms could be determined and the coolant flows controlled based on these measurements, in the sense that, for example, in the situation according to FIG. 3, the coolant flow on the left throttled and intensified on the right. However, since, as mentioned above, the situation does not have to be the same over the entire width of the cast belt or length of the rolls, the automatic mode of operation is like it is advantageous in that the right influence is effective locally at every point. The arrangement shown, where the cast strip is guided vertically from top to bottom, should be the most advantageous solution. However, it is possible to use the method on which the invention is based in any casting direction. If the pouring direction is not vertical, the coolant nozzles or the coolant flows can be dimensioned differently in order to compensate for the influence of the strip weight. 15 can also be inserted sealing strips, preferably made of rubber-elastic material. or labyrinth seals of known type can also be provided.

Claims (9)

1. A method of twin-roll casting whereby metal (5) is continuously cast between cooled, counter rotating rolls (1, 2) and leaves the gap (3) between the rolls in form of a solidified strip (6), characterized in that a flow of cooling fluid (21) is applied along the surface of the roll in the direction of the roll gap (3) and on both sides of the cast strip and is drained off in the direction of the cast strip (6) along the latter to the effect that sticking of the strip (6) to one of the rolls (1, 2) results in less intense cooling at the side where the band sticks and more intense cooling at the opposite side so that thermal stresses asymetrical with respect to the middle line occur in the strip (6) and cause a bending moment which gives rise to a detachment of the strip from a sticking roll.

2. A method according to claim 1, characterized in that the cooling fluid is accumulated, when flowing off, at each side by a gap (20a, 20b) limited by a nozzle wall (8) and the strip (6). the degree of accumulation being in each case dependent on the position of the band (6).

3. A method according to claim 1 or 2, characterized in that the cast strip (6) comes out downwards between the rolls (1, 2) while the cooling fluid is fed upwards.

4. A method according to one of the claims 1-3, characterized in that determined parameters, e. g. the position of the casted band (6), the temperatures or pressures in the cooling fluid or similar are measured and the streams of cooling fluids are individually controlled as a function of the measuring results.

5. An installation for twin-roll casting for carrying out the method of claim 1, characterized in that to each roll (1, 2) is associated a cooling fluid nozzle (7) the outlet (20) of which is correspondingly directed into the space between a roll (1, 2), one side of the cast strip (6) and the narrow gap (3) between the rolls (1, 2).

6. An installation according to claim 5, characterized in that each roll (1, 2) forms a wall of a nozzle to which front walls (10) as well as side walls (8. 9) of a nozzle body (7) are adjoined, each nozzle outlet (18) is formed by a roll (1. 2) and the adjoining side wall (8).

7. An installation according to claim 6, characterized in that sealing rods (14, 15) are loosely disposed in grooves (13) of the edges of the front walls (10) and of one of the side walls (9).

8. An installation according to claim 6 or 7, characterized in that between the side walls (8) of the two nozzle bodies (7) a gap is formed for the passage of the cast strip (6), an outlet gap (20) for the cooling fluid remaining in each case between a side wall (8) and the casted band (6) and the outlet gaps (20) being dimensioned in such a way that the cooling fluid is under pressure in the mentioned spaces.

Images